The present invention relates to a bolometer.
A bolometer is a device for measuring electromagnetic radiation of a certain frequency. It includes an absorber, which converts the electromagnetic radiation to heat, and a thermometer. In dependence on a thermal capacity of the material, there is a direct connection between a quantity of absorbed radiation and a resulting increase in temperature. Therefore, the temperature may serve as a measure for an intensity of incident radiation. What is of particular interest is bolometers for measuring infrared radiation, where most bolometers exhibit a maximum of sensitivity.
In engineering, a bolometer may be used as an infrared sensor, an image sensor for infrared binoculars, an infrared camera or a thermal image camera. Bolometers may be employed as an individual sensor or may be combined to form a sensor row and a two-dimensional array (microbolometer array).
In the sensor, a thin layer is arranged in a thermally insulated manner, e.g. suspended in the form of a membrane. The infrared radiation is absorbed in this membrane, which raises its temperature as a result. If this membrane consists of a material with a finite electric resistance, this electric resistance will change depending on the temperature rise and a temperature coefficient of the resistance. Further details are described in the following document: http://www-leti.cea.fr/commun/AR-1403/T5-Photodetection/25-J-Ltissot.pdf. Alternatively, the membrane is an insulator (silicon oxide or silicon nitride), onto which the resistor was deposited as a further thin layer. In other embodiments, insulation layers and an absorber layer are applied in addition to the resistor layer.
For the sensitivity of the bolometer, it is important that the electric resistance change rapidly and significantly along with the temperature. The temperature dependence of metal layer resistors is linear. Semiconductors used as resistor material exhibit exponential dependence. Dependence of the same magnitude may be expected from diodes used as thermal detectors, which have a current-voltage characteristic curve according to:ID=I0·(Exp{eUD/kT}−1),wherein T is the temperature, k is the Boltzmann constant, e is the electric charge, ID and UD designate a current intensity and voltage in the diode, and I0 is a voltage-independent constant.
Bolometers arranged in rows or arrays are nowadays typically fabricated on silicon substrates by means of surface micromechanics using methods of microsystem engineering. The reference here is to microbolometer arrays.
A wavelength of the infrared radiation to be detected ranges from 8-14 μm, as in this wavelength range, there are radiating solids having approx. room temperature (300 K). The wavelength range of 3-5 μm is also of interest because of the diaphanous atmospheric window.
FIG. 3 exemplarily shows a bolometer. It includes a thermometer layer of amorphous silicon, which is stretched as a membrane between two spacers 820 and 830 at the corners thereof. Below this membrane, a reflector layer 840 is positioned at a distance of approx. 2.5 μm. The thermometer layer, or membrane, has a thickness of approx. 0.1 μm. The spacers 820 and 830 do not only fix the membrane 810 above the reflector layer 840 on opposite sides, but also establish an electrical contact between the respective end of the thermometer layer 810 and an underlying circuit (not shown), such as e.g. to a readout integrated circuit (ROIC) via a metal pad. For obtaining maximum thermal sensibility, the thermometer layer 810 is, at the suspended ends, formed such that it is connected to the spacers via narrow portions 850 and 860, so that thermal insulation of the thermometer layer 810 may be achieved.
Compared to other (photonic) infrared detectors, it is a substantial advantage of thermal bolometers that they may be operated at room temperature, that is without cooling.
Good diodes based on anorganic semiconductor materials may as yet only be fabricated in monocrystalline silicon, not, however, in amorphous silicon or other semiconductor materials hitherto employed in a bolometer (vanadium oxide, amorphous or polycrystalline Si, Ge or SiGe). The use of monocrystalline silicon on a thermally insulated membrane is possible e.g. by a silicon-on-insulator technique or by undercutting diodes in the silicon substrate. These techniques are described in: T. Ishikawa et al.: “Performance of 320×320 Uncooled IRFPA with SOI Diode Detectors”, Proc. SPIE, vol. 4130, pp. 152-159, (1400), or in: P. Neuzil, Y. Liu, H.-F. Feng, and W. Zeng: “Micromachined Bolometer with Single-Crystal Silicon Diode as Temperature Sensor”, IEEE Electron. Dev. Letters, vol. 26, no. 5, May 2005, pp. 320-322.
Drawbacks of the existing technology include, for example, that, in the use of anorganic semiconductor materials, active (CMOS) control and amplifier elements may not be integrated underneath the sensor structure. Co-integrating these elements next to the detector, which might be possible, will result in a drastic reduction of the filling factor (detector area versus total area), thereby raising the chip area and the detector cost. In addition, these structures may not be used for fabricating an absorber having a resistor layer amounting to the spreading resistance of an electromagnetic wave in air (377 Ω/□) and being arranged above a reflector at a distance of λ/4 (of approx. 2.5 μm, at a wavelength of 10 μm).
In the paper: J.-J. Brissot, F. Desvignes, and R. Martres: “Organic Semiconductor Bolometric Target for Infrared Imaging Tubes”, IEEE Trans. Electron. Dev., vol. 20, no. 7, July 1973, pp. 613-620, (1973), a bolometric target from an undoped organic semiconductor is utilized as a temperature-dependent resistor in an infrared vidicon.
The use of undoped organic semiconductor layers as used in the paper just cited, in turn involves the drawback that this layer has a very high impedance (1014 Ωcm at 70° C.) and both sides are contacted extensively.